CRUSHING AND GRINDING 205
Grinding pans constitute one of the oldest types of fine-reduction machinery. There are many different types, but probably the best known of these is the Wheeler pan, which was formerly very widely used in amalgamating processes in silver mills. These machines comprise a pan or tub 3, 4 or 5 ft. in diameter, made of steel or wood, with a comparatively shallow depth, ordinarily approximating 3 or 4 ft. The bottom of the pan is equipped with hard-steel or iron plates upon which a muller operated through a central shaft grinds the ore. The muller is equipped with grinding shoes, adjustable so that its distance from the bottom may be any required measurement. The muller revolves, rubbing its shoes against the hard iron bottom of the pan. The grinding is performed by the abrasion of the material between these two hard surfaces.
In operation machines of this type produce very finely divided material, but since they involve rapid and costly destruction of the grinding surfaces, they are not considered economical for present-day use. Their capacity is not great, an ordinary 5-ft. pan being capable of handling 10 to 15 tons per day when reducing from 10 or 15 mesh to an average of 60- to 80-mesh material. The power required is from 10 to 20 hp., depending upon the character of the rock and fineness of the product.
Edge runners may be typified by the mill known as the Chilean mill. There are many types.of this machine, the usual type having straight grinding faces, but there are some variations having their grinding faces curved, one convex and the other concave. The Chilean mill consists essentially of a bottom ring die over which rolls a heavy wheel, actuated through a central shaft. The type is an old one, having been originally used in crude form by the early Spanish miners in Spanish America. For comparatively coarse work the, machine is made with a rolling diameter comparatively large, reaching 6 or 7 ft. This machine is operated at comparatively slow speeds, usually 12 to 18 r.p.m., or sometimes even less. For fine reduction, the modern type of Chilean mills has a running diameter of about 4 or 5 ft. and is operated at speeds up to 30 or 35 r.p.m.; in this case the wheels are smaller and lighter than with the large diameter mill. Chilean mills are operated either wet or dry. Their work is performed largely by rolling action, but one variation of the mill adds somewhat to its capacity by providing the factor of abrasion. This is by swinging the roller to what is essentially a crank. The horizontal shaft connecting with the driving mechanism of the center of the mill, is provided with an offset of 2 to 4 in., the shaft of the roller being connected to the offset. This gives to the rolling action a sliding or grinding effect so that the mill is dragged over the die in addition to rolling over it.
This variation is known as the Mantey offset. It increases the fineness of the product, but introduces the factor of abrasion which increases the wear on the grinding surface and consequently the cost of operation.
The power required for operating Chilean mills is about five- to seven-tenths of a ton per horsepower-hour when producing a fine product. The capacity varies according to the work done. On comparatively coarse grinding, with slow-speed mills, in the neighborhood of 20 to 25 tons per hour may be reduced, while with high speeds this capacity is increased.
Ring-roll grinding machines are of the type in which the grinding anvil is a horizontal ring. The mullers swung from a spider attached to a central shaft, press against this vertical grinding surface. The Huntington mill is the best